In an
F
command, rather than specifying the name of a file, specify
the name of a directory or a device such as
/dev/null
. Does
sendmail
care? How does it handle (or not handle) nonfiles?

Use
/usr/dict/words
as a class file. Does reading a huge file
slow
sendmail
down noticeably? Does this suggest that a database
might be more efficient for some needs?

Are there security concerns surrounding use of the
F
command? Imagine ways in which a cracker might exploit a careless use of the
F
command to compromise your system. Skip ahead to
Section 22.4.2, "The F Command - File Form"
to explore this question further.